The invention relates to a method for locally removing coatings from components, in particular gas-turbine or aircraft-engine components which have one or more coatings at least in certain areas.
Components of internal combustion engines, such as gas turbines, have a wide variety of coatings. For removing such coatings, for example after damage during operation, so-called immersion methods are known. In these, the component is completely immersed in an electrochemical bath for removing coatings, so that not only the coated surfaces but all the surfaces of the component (base material, coatings etc.) are subjected to a material attack. A selective immersion of the coated areas of the component is often restricted by the component geometry, i.e. the lack of accessibility to the coating. The use of covers for the uncoated areas of the component leads to interfacial reactions due to restricted wettability, and consequently to an uncontrolled attack on the uncoated areas.
An object of the present invention is to provide a method with which components provided with coatings can have their coatings removed selectively or restricted to local areas. Moreover, a device for selectively or locally removing coatings from components is to be provided.
These and other objects and advantages are achieved by the method according to the invention in that a component having a coating on at least certain areas is provided, an absorbent medium is provided, the medium is supplied with a coating removal liquid and the medium containing the coating removal liquid is brought into contact with the area of the component from which the coating is to be removed.
Preferably, coatings can be removed from components without uncoated surfaces or the base material of the component being attacked by the coating removal liquid or stripping liquid. Desirably, the coatings are not removed completely, but only the damaged areas. If the component also has other coatings, not all the coatings have to be removed, but preferably just the damaged ones. This leads to savings of time and costs in the repair procedure. In addition, less coating removal liquid is required than in the case of the immersion method. Furthermore, no additional covers are required for areas of the component from which coatings are not to be removed. The local removal of coatings according to the invention has the effect that the base material underneath the coating and the base material in uncoated areas of the coating is not attacked, or only within the permissible limit values.
In another embodiment, absorbent cotton or a sponge or a porous material which may be ceramic or synthetic, for example synthetic fibrous material, may be provided as the medium. The medium absorbs the liquid and stores it, so that coatings are removed from the component when it comes into contact with the medium. It is a general prerequisite for this invention that the medium is resistant to the coating removal liquid. The medium is in this case continuously supplied with coating removal liquid.
The coating removal liquid can preferably be continuously supplied to the medium, for example by an inflow or drip feeding device and corresponding controller, whereby the area of the component from which the coating is to be removed is constantly wetted with coating removal liquid and has the effect of attacking or dissolving the coating. The constant flow, i.e. inflow and outflow, of the liquid ensures that the composition of the coating removal liquid which wets the area from which the coating is to be removed remains substantially the same.
The contact between the medium containing the coating removal liquid and the area of the component from which the coating is to be removed can be maintained over a period of time which may last from a few minutes to several hours, depending on the thickness and type of coating, and in particular may last for 12 to 48 hours.
The medium can preferably be held in a liquid-impermeable receptacle, the shape of which can be formed in a way corresponding to the shape of the component in its area from which the coating is to be removed. The receptacle is preferably formed in such a way that the medium between the receptacle and the area of the component from which the coating is to be removed is in close contact with the area of the component from which the coating is to be removed during the coating process.
The receptacle may have at least one inlet, by which the medium is supplied with coating removal liquid.
Depending on the type of coating, an acid or a caustic solution or salt solution may be provided as the coating removal liquid. The local removal of coating may take place chemically or electrochemically, so that a chemically active or electrochemically active coating removal liquid may be provided as the coating removal liquid.
The method may be used on metallic components, provided with a coating at least in certain areas, of stationary gas turbines or aircraft engines, such as for example a blade or an integrally bladed rotor carrier. A wide variety of coatings, such as coatings protecting against high-temperature corrosion or oxidation or coatings protecting against wear, such as blade tip claddings containing hard particles, can be selectively or locally removed in this way.
By heating the coating removal liquid above room temperature, the time period of the coating removal operation can be reduced, it being possible to achieve good results with shorter coating removal time periods at a temperature of the coating removal liquid above 40° C., and in particular in the range from 40 to 60° C.
The medium and the area of the component from which the coating is to be removed may be moved in relation to each other. In this way, fine residues on the coating can be removed or surfaces of the coating still to be removed can be exposed to improve the superficial reactions. Moreover, the area from which the coating is to be removed is wetted more uniformly with the coating removal liquid and its constant replacement is improved. This leads to a reduction of the coating removal time period.
In another embodiment, an object of the invention is achieved by way of at least one holding device for a component having a coating at least in certain areas, and at least one receiving device for an absorbent medium containing a coating removal liquid, the holding device and/or the receiving device being positionable in such a way that the medium containing the coating removal liquid makes contact with the area of the component from which the coating is to be removed.
The receiving device can preferably have at least one inlet for the coating removal liquid and at least one outlet for liquid, so that a constant composition of the coating removal liquid coming into contact with the areas from which coatings are to be removed is ensured.
Furthermore, the device can preferably have a number of receiving devices for the simultaneous removal of a number of coatings or coating areas of a component or a number of components. For example, in the case of an integrally bladed rotor carrier of a gas turbine, the blade tip claddings of a number of blades can be removed simultaneously, without the uncoated surfaces of the rotor carrier being attacked by the coating removal liquid. Similarly, the blade tip claddings of a number of individual blades of gas turbines can be removed by the corresponding number of receiving devices simultaneously.
In yet another embodiment involving an electrochemical coating removal method, the component may be anodically connected or subjected at times, for example at regular intervals, to anodic-cathodic pole reversal or pulsing. The pulsing/pole reversal may also take place over the entire coating removal time period. The pulsing/pole reversal allows specific atoms or molecules, such as for example hydrogen, to be deposited on the surface of the component area from which the coating is to be removed, where they react and speed up the coating removal process. Furthermore, in the case of electrochemical coating removal, the controlling of the current and voltage is adapted to the base material of the component and to the coating to be removed (type, thickness, size).
Further refinements of the invention are described in the subclaims.